Update WebKit revision in DEPS to pick up my fix for purify.

Review URL: http://codereview.chromium.org/53085

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@12532 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -19,7 +19,7 @@ deps = {
     "http://googletest.googlecode.com/svn/trunk@214",
 
   "src/third_party/WebKit":
-    "/trunk/deps/third_party/WebKit@12511",
+    "/trunk/deps/third_party/WebKit@12531",
 
   "src/third_party/icu38":
     "/trunk/deps/third_party/icu38@12390",